The present invention relates generally to traction control systems for vehicles and, more particularly, to a method for evaluating the effectiveness of a tractive effort control system in a vehicle such as an ac electric motor propelled locomotive.
In a modern conventional diesel-electric locomotive, a thermal prime mover (typically a 16-cylinder turbocharged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of electric traction motors whose rotors are coupled through speed-reducing gearing to the respective axle-wheel sets of the locomotive. The generator typically comprises a main 3-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase armature windings on the stator of the alternator. These voltages are rectified and applied via a DC link to one or more inverters where the DC voltage is inverted to AC and applied to AC traction motors.
In normal motoring operation, the propulsion system of a diesel-electric locomotive is so controlled as to establish a balanced steady-state condition wherein the engine-driven alternator produces, for each discrete position of a throttle handle, a substantially constant, optimum amount of electrical power for the traction motors. In practice, suitable means are provided for overriding normal operation of the propulsion controls and reducing engine load in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capability of the engine at whatever engine speed the throttle is commanding.
One factor affecting traction performance is the creep level of the locomotive's traction control subsystem. Accordingly, in order to maximize traction performance, it is desirable to separately control the allowable creep level of each individual axle.
Another factor affecting traction performance is the level of torsional resonant vibration in the mechanical drive train, which comprises a locomotive axle and its associated two wheels, the motor to axle gearbox, the induction motor, and the induction motor drive. In particular, during operation in certain regions of the adhesion characteristic curve, the mechanical drive train may experience a net negative damping which produces severe vibration levels at the system's natural frequencies. As is well-known, an adhesion characteristic curve graphically represents coefficient of friction versus percentage creep. At 0% creep, maximum damping on the mechanical system is represented. As the creep level increases in the portion of the characteristic curve to the left of its peak, the damping effect on the mechanical system decreases to a value of zero at the peak. For values of creep to the right of the peak, the damping provided to the mechanical system becomes a larger negative number.
U.S. Pat. No. 5,841,254 discloses one form of tractive effort maximizing and vibration control system for ac electric motor propelled locomotives. In general, such a traction control system for an ac locomotive optimizes traction performance by separately controlling the allowable creep level of each individual axle and by minimizing torsional vibration per axle. The traction control system comprises a torque maximizer and a torsional vibration detector. The torque maximizer measures traction system performance levels and determines the desired torque maximizer state for maximizing traction performance of each individual axle. The torsional vibration detector digitally processes estimated torque feedback of each traction motor in order to detect an unacceptable level of torsional vibration. The outputs of the torque maximizer and the torsional vibration detector are provided to a creep modulator which processes these inputs in order to control the operating creep level of each locomotive axle. As a result, traction performance is improved while minimizing torsional vibration and operating noise levels due to wheel/rail squeal. Note also that the control system may be used in other ac induction motor propelled vehicles such as, for example, off-highway, earth-moving vehicles and transit cars.
The development of traction control systems is a continuous process with various forms of control systems being developed. One problem with the developed systems is determining whether such new systems function as well as or better than other systems. In a typical test environment, the general practice is to operate the vehicle on a test track using a standard system, remove the standard system, install the new system and repeat the operation. During each operation, the level of tractive effort produced is measured. What such a process fails to accommodate are changing track conditions between tests of the standard and new system. For example, in ideal weather, wheel adhesion on a rail track will improve as the track is used. However, any variation in weather can affect track conditions. Such variations can include snow or rain. Further, if tests are run days apart, corrosion on the track will affect adhesion. Such changes in track adhesion characteristics can easily bias measurements of control system effectiveness. Accordingly, it would be desirable to minimize the effects of changing track conditions during control system evaluation.